Unity - Mirror Networking by First Gear Games - Short tutorials that explain the basics and shows example code for Mirror. Also includes paid tutorials with more advanced features and extensions to mirror.
Mirror Multiplayer by Dapper Dino - Tutorials that explain how to implement different features using Mirror.
Unity + Mirror Networking Tutorials by Shrine Wars - Explains the basics about Networking with Unity and Mirror.
Mirror Networking Tutorials by Jared Brandjes - Long tutorials explaining how to set up lobbies and matchmaking with Mirror.